1. Technical Field of the Invention
The present invention relates generally to a load driving circuit designed to drive a current flowing through an electrical load using an insulated gate transistor, and more particularly to an overcurrent protection structure of such a load driving circuit equipped with with an overcurrent detector and an overcurrent limiter.
2. Background Art
FIG. 6 shows an example of an electric structure of a conventional load driving circuit equipped with MOSFETs.
The load driving circuit 1 is fabricated on a control IC installed in, for example, an electronic control unit (ECU) of automotive vehicles and works to drive an electrical load 3 such as a relay coil or an LED disposed between an output terminal 1a of the IC and a battery 2.
Disposed between the output terminal 1a and a ground terminal 1b are an FET4 through which a main current flows and an FET 5 working to measure the current. A resistor 6 is connected between the FET 5 and the ground terminal 1b. The FETs 4 and 5 connect at gates thereof to each other. Zener diodes 7 and 8 are connected to the gates of the FETs 4 and 7 in an illustrated orientation. The load driving circuit 1 also includes an operational amplifier 9 which works to control the voltage at the gates of the FETs 4 and 5 as a function of a difference between the voltage developed across ends of the resistor 6 and a reference voltage (i.e., a threshold current).
The zener diodes 7 and 8 work to protect the gates of the FETs 4 and 5 from an overvoltage. If a condition that an unusual rise in voltage of the battery 2 or a drop in impedance of the load 2 will cause an overcurrent exceeding the threshold current to flow is encountered, the operational amplifier 9 serves to bring the current into agreement with the threshold current to protect the FET 4 from the overcurrent. An output of the operational amplifier 9 is used as an overcurrent detection signal.
A static charge may be added to the output terminal 1a or the ground terminal 1b through a line extending from the battery 2 to the load 3. In order to avoid this, an external ESD (electrostatic discharge) protective capacitor 10 is joined between the output terminal 1a and the ground terminal 1b to enhance the ability of the load driving circuit 1 to withstand the ESD. The addition of the capacitor 10, however, causes all charge stored in the capacitor 10 to be released through the FET 4 when the FET 4 is switched from an on-state to an off-state, so that an excessive discharged current flows in the load driving circuit 1, which causes the operational amplifier 9 to output the overcurrent detection signal in error. Specifically, the system is responsive to the covercurrent detection signal outputted from the operational amplifier 9 to determine in error that the overcurrent is flowing through the load driving circuit 1.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a load driving circuit designed to increase the accuracy of detecting an overcurrent flow arising from an unusual condition of an electrical load or a power supply and/or to protect circuit components from the overcurrent.
According to one aspect of the invention, there is provided an electrical load driving circuit which may be employed in an electronic control unit for automotive vehicles. The electrical load driving circuit comprises: (a) an insulated gate transistor installed in an electrical line extending between a power supply and an electrical load, the insulated gate transistor working to output a current as a function of a control voltage applied across a control terminal and a first terminal thereof; (b) a current measuring circuit working to measure a current flowing through the insulated gate transistor; (c) a control voltage limiting circuit disposed between the control terminal and the first terminal, the control voltage limiting circuit including a control circuit which and a switching circuit, the control circuit working to limit a voltage developed across the control and first terminals selectively to one of a first controlled voltage and a second controlled voltage lower in level than the first controlled voltage, the switching circuit being disposed between the control terminal and the first terminal and closed in response to input of a close control signal to establish electric communication between the control terminal and the first terminal to limit the voltage developed across the control and first terminals to the second controlled voltage through the control circuit; and (d) a comparator working to compare the current as measured by the current measuring circuit with a threshold current that is selected within a given current range smaller than a maximum current allowed to flow through the insulated gate transistor. When the measured current is greater than the threshold current, the comparator provides the close control signal to close the switching circuit of the second control voltage limiting circuit.
Specifically, when the current flowing through the insulated gate transistor exceeds the threshold current, the comparator works to close the switching circuit to limit the voltage developed across the control terminal and the first terminal of the insulated gate transistor to the second controlled voltage. The current flowing through the insulated gate transistor limited by the second controlled voltage (i.e., a saturation current in a case where the insulated gate transistor is a MOSFET) is greater than the threshold current and does not affect the transistor undesirably. The electrical load driving circuit works to detect occurrence of the overcurrent based on a difference between those currents to protect the transistor from the overcurrent.
In the preferred mode of the invention, the electrical load driving circuit further comprises an overcurrent determining circuit which works to provide an overcurent signal indicating occurrence of the overcurrent when the comparator continues to output the close control signal after the elapse of a given period of time from output of the close control signal from the comparator. This enables the overcurrent which results from a malfunction of the load or a power supply to be detected accurately. Specifically, the overcurrent determining circuit determines the occurrence of overcurrent only when the covercurrent resulting from the malfunction of the load or the power supply continues to flow.
A capacitive load is further provided which is joined between the first terminal and a second terminal of the insulated gate transistor. The overcurrent determining circuit is designed to make a decision to output the overcurrent signal after a time has been reached which is required to release a charge from the capacitive load.
The electrical load driving circuit may also include a turn-off circuit which turns off the insulated gate transistor in response to output of the overcurrent signal from the overcurrent determining circuit, thereby avoiding breakage of the insulated gate transistor.
The current measuring circuit includes a measuring transistor which connects at a control terminal thereof with the control terminal of the insulated gate transistor and has a given cell ratio between itself and the insulated gate transistor and a measuring resistor working to measure the current flowing through the measuring transistor. This permits the current provided by the cell ratio to flow through the insulated gate transistor.
The electronic load driving circuit may further comprise an electrostatic discharge protective capacitor disposed between the first terminal and the second terminal of the insulated gate transistor.